1. Field of the Invention
The present invention relates to an ink jet ink, an ink cartridge containing the ink jet ink, and an ink jet recording method.
2. Description of the Related Art
In some ink jet inks, a pigment is dispersed using a polymer. Also in such polymer-dispersed pigment inks, there has recently been a demand for high ink reliability (including ejection stability and ejection accuracy), image quality (including high optical density), and image fastness (including scratch resistance and highlighter resistance). In order to improve these characteristics, various polymer-dispersed pigment inks containing a polyurethane polymer have been studied (Japanese Patent Laid-Open No. 2005-290044 and Japanese Patent Laid-Open No. 2008-266595). Japanese Patent Laid-Open No. 2005-290044 discloses an ink that contains a pigment dispersed using a polyurethane polymer. Japanese Patent Laid-Open No. 2008-266595 discloses an ink that contains a polymer-dispersed pigment and a polyurethane polymer having a carboxy group.
Inks containing a polyurethane polymer have been studied in which the number of urea bonds in the polyurethane polymer is specified in consideration of the correlation between the urea bond and the characteristics of the ink (Japanese Patent Laid-Open No. 9-291242 and Japanese Patent Laid-Open No. 2004-285344). Japanese Patent Laid-Open No. 9-291242 discloses an aqueous printing ink that contains a polyurethane polymer having a specific ratio of the urethane bond to the urea bond. The aqueous printing ink has improved adhesion to a plastic film and improved laminate strength. Japanese Patent Laid-Open No. 2004-285344 discloses an ink that contains an acrylic-polymer-dispersed pigment and a polyurethane polymer having no urea bond. The ink has improved storage stability and improved gloss.
The present inventors found that conventional pigment inks containing a polyurethane polymer have improved ink reliability and image fastness but still do not sufficiently satisfy the requirements.
The present inventors found that the use of a pigment dispersed using a polyurethane polymer as described in Japanese Patent Laid-Open No. 2005-290044 results in poor ink ejection stability. This is because the polyurethane polymer described in Japanese Patent Laid-Open No. 2005-290044 is not hydrophilic enough to disperse the pigment. The polyurethane polymer described in Japanese Patent Laid-Open No. 2005-290044 contains a nonionic component derived from a polyether polyol. The present inventors found that a pigment dispersed using a polymer having such a structure aggregates slowly on a recording medium, resulting in poor color developability.
In Japanese Patent Laid-Open No. 2008-266595, the polyurethane polymer is synthesized only using a polyisocyanate having a ring structure, such as isophorone diisocyanate or 4,4-dicyclohexylmethane diisocyanate. It was found that such a polyurethane polymer has a large number of hard segments described below because of an interaction between the ring structures and hydrophobic interaction and becomes excessively rigid, resulting in poor image fastness. Japanese Patent Laid-Open No. 2008-266595 also discloses a polyurethane polymer synthesized using a poly(propylene glycol) having a molecular weight of 400. However, it was found that the short molecular chain of the polyol results in low flexibility of the polyurethane polymer relative to its strength, resulting in low scratch resistance and highlighter resistance of an image.
In the polyurethane polymer disclosed in Japanese Patent Laid-Open No. 9-291242, the molar ratio of the urethane bond to the urea bond is in the range of 80/20 to 50/50. The present inventors found that the ejection of such an ink containing a polyurethane polymer as disclosed in Japanese Patent Laid-Open No. 9-291242 through an ink jet recording head results in the deposition of a polymer component of the ink on a surface of the recording head on which the ejection ports are disposed (hereinafter referred to as a “face”). This phenomenon is referred to as “face wetting”. The polymer component deposited on the face may change the intended flight direction of an ejected ink droplet. This deflection may be small immediately after the ejection of the ink droplet but increase before reaching a recording medium, causing a problem of the deflection of some dots in the image (hereinafter referred to as “image deflection”). This image deflection occurs not only in a thermal ink jet method but also in an ink jet method involving the use of a piezoelectric element.
The polyurethane polymer disclosed in Japanese Patent Laid-Open No. 2004-285344 has no urea bond. Thus, the aqueous pigment ink disclosed in Japanese Patent Laid-Open No. 2004-285344 rarely causes image deflection. However, as in Japanese Patent Laid-Open No. 2008-266595, the polyurethane polymer is synthesized only using a polyisocyanate having a ring structure. This results in poor image fastness.